Composite structures, especially structures made of carbon fibers and thermosetting or thermoplastic resin, are gaining rapid acceptance in aircraft primary structure and other applications that require a high strength to weight ratio and/or high stiffness to weight ratio. For example, the main structure of the Boeing™ 787 is reported to be made up of more than 50% carbon-epoxy composites.
Some of the most demanding aircraft structures including wings, propellers, and rotor blades require high strength and high stiffness from long and thin structures, called high aspect ratio structures; these structures are further required to be lightweight. Such structures are well-suited to carbon composite construction. The majority of such aircraft composite structures are manufactured by placing carbon fibers that are pre-impregnated with epoxy resin (pre-preg in the industry vernacular) in molds, and then curing the composites in ovens or pressurized ovens (called autoclaves).
Manufacturing composite structures often requires complex manufacturing tools, with relatively high labor content and other costs. Additionally, when such structures are designed to the minimum required thickness to provide the specified strength and stiffness, the reduced thickness makes the structure prone to buckling instability. Design against buckling requires either increased thickness (entailing higher weight and cost) or additional support through adoption of sandwich construction using a lightweight core material. For most high aspect ratio structures, including helicopter rotor blades, this sandwich construction is impossible or impractical to achieve in the current manufacturing process, as the curing of an enclosed box form at high aspect ratio requires complex tooling or creates inaccessible voids.
Thus, there is still a need for improved methods of producing lightweight, enclosed high aspect ratio composite structures that include resistance against buckling.